1. Field of the Invention
This invention relates to an overload detecting circuit for an amplifier and, particularly to an overload detecting circuit which is preferably used for a PWM amplifier to produce an amplified output by means of pulse width modulation in response to an input signal.
2. Description of the Prior Art
A transistor output amplifier being of class-D type is converted into a pulse-width-modulated signal (PWM signal), a switching element being controlled in ON-OFF state in response to the PWM signal, and the switching output being demodulated through a filter to derive an amplified output signal.
When, for example, a speaker unit having a lower input impedance than a rating of the amplifier is connected as a load, or the output terminals of the amplifier are shorted, the amplifier runs into an overload condition. The current flowing through the switching element exceeds rated value of the amplifier, so that the switching device is in danger of being broken down.
Therefore, the amplifier needs an overload detecting circuit for protecting the switching element. A such conventional overload detecting circuit operates in the manner that the current flowing through the switching element is detected, and when the detecting current exceeds a rated value, the input signal is attenuated or the PWM signal to the switching element is cut off. The detecting circuit detects the current flowing through the switching element. The detecting current is larger than the rated maximum permissible current of the switching element. Thus, when such a trouble that the output terminals are shorted occurs, a current larger than the rated maximum permissible current flows through the switching element. The current causes a large quantity of loss such as generation of heat, until an overload protecting means operates.
Further, the filter to which the output of the switching element is supplied to demodulate the PWM signal, consists of an inductor and a capacitor. The switching current induces a counter electromotive force, which causes a reverse current. The reverse current flows through a discharge diode. Thus, in the amplifier using such an overload detecting circuit, the reverse current flowing through the diode becomes larger in accordance with the maximum output current of the amplifier under the overload condition thereof, which results in generation of a large quantity of power loss in the diode.